This invention relates to apparatus for use in a liquid alkali metal environment.
The use of liquid alkali metals such as sodium in industry is now quite common. For example, in the nuclear reactor industry liquid sodium is used as a vehicle to convey heat energy from a nuclear reactor core to heat exchange apparatus wherein steam is generated and superheated. Because of the hostile nature of the material particularly when operating in the temperature range of 500.degree.-600.degree. C., wear, fretting and high coefficients of friction between bearing surfaces present a serious problem. Heat exchangers for use with sodium are particularly susceptible to this problem. In one known sodium/water heat exchanger a 9% Cr 1% Mo stainless steel has been selected for the heat exchange tubes because this material offers high resistance to stress corrosion cracking, pitting corrosion and to decarburisation but where the tubes make bearing contact with complementary bearing surfaces giving lateral support to the tubes there is a tendency for transfer of the alloy steel to the complementary bearing surface. Several materials have been used or suggested for the complementary bearing surfaces, for example stabilised 21/4 Cr 1 Mo stainless steel, nickel base alloys such as Inconel 718 (Inconel is a registered trade mark), stellite and chromium carbide but none of these bearing surface materials are satisfactory when used with stainless steel tubes.